P OSTHARVEST BIOLOGY & TECHNOLOGY
HORTSCIENCE 30(5):1052–1053. 1995.
Reduced Heat Shock Period of
‘Sharwil’ Avocado for Cold Tolerance
in Quarantine Cold Treatment
Kate A. Nishijima, Harvey T. Chan, Jr., Suzanne S. Sanxter, and
Edward S. Linse
U.S. Department of Agriculture, Agricultural Research Service, P.O. Box
4459, Hilo, HI 96720
Additional index words. Persea americana, precondition, heat treatment, cold storage,
chilling injury, fruit fly, heat shock
Abstract. A reduced heat shock period for ‘Sharwil’ avocado (Persea americana Mill.)
before quarantine cold treatment is described. The shortened heat pretreatment period of
8 to 12 hours, rather than the originally recommended 18 hours at 38C, is effective in
reducing chilling injury symptoms when the pulp is at ≤2.2C during 16 days of storage. The
reduced durations and the range of pretreatment hours affords packinghouses greater
efficiency and more flexibility and will reduce handling costs relative to the longer
exposure.
‘Sharwil’ avocado fruit is a host for tephritid
fruit flies when the trees are under drought
conditions (Liquido et al., 1994). Therefore,
avocados have been banned for further export
from Hawaii to the U.S. mainland until a
quarantine treatment is approved by the Animal and Plant Health Inspection Service
(APHIS). A heat shock pretreatment (HSP)
for inducing cold tolerance in ‘Sharwil’ avocado fruit before quarantine cold treatment has
been developed (Sanxter et al., 1994). The
HSP involves heating fruit at 37 to 38C (air)
for 17 to 18 h, followed by cooling at 20C (air)
for 4 h. Avocados then were placed in 1.1C
(air) and stored for 16 days at ≤2.2C pulp
temperature. This cold temperature storage
has been approved as a quarantine treatment
for many other fruit for the disinfestation of
Mediterranean fruit fly [Ceratitis capitata
(Weidemann)] (APHIS, 1994). Our report describes reduced periods (<17 to 18 h) of heat
shock pretreatment at 38C that still induces
fruit tolerance to chilling injury during storage
at pulp temperatures of ≤2.2C. This shorter
pretreatment gives more flexibility in handling and reduces processing time.
Received for publication 14 Nov. 1994. Accepted
for publication 24 Mar. 1995. We thank Myles H.
Taniguchi for his help with statistical analysis. Reference to a company and (or) product named by the
U.S. Dept. of Agriculture is only for purposes of
information and does not imply approval or recommendation of the product to the exclusion of others
that may also be suitable. The cost of publishing this
paper was defrayed in part by the payment of page
charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to
indicate this fact.
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Materials and Methods
‘Sharwil’ avocados were harvested from
commercial orchards in the South Kona district, Island of Hawaii. Fruit were transported
by truck to the U.S. Dept. of Agriculture,
Agricultural Research Service, Tropical Fruit
and Vegetable Research Laboratory, Hilo, Hawaii, within 24 to 48 h. Fruit were culled for
defects and placed in plastic mesh bins in lots
of 20 or 40 fruit per treatment. Heat pretreatments were conducted at 37 to 38C in a 0.6-m3
incubator (model RI-23-1060-A; Revco Scientific, Asheville, N.C.) with an air flow rate
of 0.1 to 0.6 m• s–1 for 6, 8, 10, 12, or 18 h. After
heating, fruit were cooled at ≈23C for 4 h or
until pulp temperature at the seed reached
≤28C. Fruit then were transferred to a walk-in
refrigerator (Forma Scientific, Marietta, Ohio)
at 1.1C. When pulp temperatures at the seed
reached ≤2.2C, fruit were placed in perforated
(holes 6 mm in diameter, centered every 14 cm
across each bag), 50 × 76 cm, 0.08-mm-thick,
polyethylene bags (Shields Bag and Printing
Co., Yakima, Wash.). The bagged fruit were
placed in vented fiberboard boxes and held at
1.1C for 16 days with pulp temperatures ≤2.2C.
Pulp and air temperatures were monitored
with a polycorder (Omnidata International,
Logan, Utah) using thermistors calibrated at
the start of each weekly experiment. The temperature profile of fruit heated for 18 h was the
same as in the previous experiments (Sanxter
et al., 1994). Control fruit were not heat shocked
and were held for 16 days at 7C in vented
fiberboard boxes without using perforated bags,
as is done commercially. Previous work indicated that fruit subjected to quarantine cold
storage without HSP were unacceptable
(Sanxter et al., 1994); therefore, this treatment
was excluded from our study.
After treatment, fruit were ripened at room
temperature (23C) and evaluated for quality at
the eating ripe stage. External quality was
based on the degree of surface discoloration,
pitting, and disease. Two experienced graders
independently evaluated external quality on a
scale of 1 to 5, where 1 = unacceptable, 2 =
marginally acceptable, 3 = acceptable/marketable, 4 = good, and 5 = excellent fruit. The
independent ratings were averaged per fruit,
and per treatment. Internal acceptability was
judged in consensus by cutting the fruit into
cross-sections between the seed and peduncle
to evaluate the vascular system and by cutting
again distally to evaluate the pulp. Fruit were
rated unacceptable internally if the pulp was
lumpy, the vascular tissue was stringy, or the
pulp or vascular tissue was more than slightly
discolored or diseased (Sanxter et al., 1994).
Data were analyzed using the G test (Sokal and
Rohlf, 1969), which is a modified χ2 test used
to analyze frequencies of occurrences in classes
or categories. In our case, frequencies in each
external quality rating category of external
acceptability (i.e., the rating scale of 1 to 5)
were analyzed to distinguish differences between treatments.
Results and Discussion
There were only slight differences between
HSPs for external quality (Table 1). There
were no significant differences between the
external ratings of fruit that were exposed to
HSP for 8, 10, or 12 h as shown by G values of
<15.57 (Table 2). The 6- and 18-h HSPs resulted in similar responses (G value slightly
less than 15.5). However, the 6- and 18-h
HSPs were significantly different from those
for 8, 10, and 12 h (Table 2). Although the 6and 12-h HSPs had the same mean external
rating (Table 1), the G test distinguished a
difference between these two based on their
rating distributions. The 10-h HSP resulted in
the highest external rating and percent internally acceptable fruit of all HSPs (Table 1).
The percentages of acceptable fruit based on
the internal condition of the fruit were similar,
and no HSP surpassed or equaled the quality of
the nontreated control fruit stored at 7C (Table
1). Results for fruit treated for 18 h were
Table 1. Effect of duration of 38C (air) heat shock
pretreatment before quarantine cold storage
(≤2.2C pulp temperature, for 16 days) on appearance of avocado fruit. Control fruit (no heat
shock) were held at 7C for 16 days and all fruit
were ripened at room temperature (≈23C).
Fruit
Mean
internally
Total
Hours
external acceptable No.
no.
heat
ratingz
(%)y
tests fruit
7C (control) 3.6
88.3
3
80
6
2.9
72.8
4
159
8
3.0
71.0
9
319
10
3.1
78.6
8
299
12
2.9
75.1
8
297
18
2.7
68.9
8
259
z
External appearance was based on the following
scale: 1 = unacceptable, 2 = marginally acceptable,
3 = acceptable/marketable, 4 = good, 5 = excellent.
y
Fruit were acceptable internally if pulp lacked
defects or disease (see text).
HORTSCIENCE, VOL. 30(5), AUGUST 1995
Table 2. External quality (frequency in quality categories) of ‘Sharwil’ avocados as a function of heat shock pretreatment (38C in air) duration (hours) before
quarantine cold storage. Control fruit (no heat shock) were held at 7C. After 16 days of storage (1.1 or 7C in air), all fruit were held at room temperature (≈23C)
until ripe.
No. fruit
Hours
Quality rating category
G-value matrix (df = 8)z
heat
1
1.5
2
2.5
3
3.5
4
4.5
5
Total Means 7C control
6
8
10
12
7C (control)
1
0
1
2
27
5
37
3
4
80
8.89
6
0
5
22
11
101
6
13
1
0
159 17.67
76.47
8
5
2
26
27
184
32
37
6
0
319 35.44
66.60
20.97
10
4
4
17
27
171
39
28
7
2
299 33.22
65.78
28.48 7.31
12
6
4
31
40
159
21
25
10
1
297 33.0
76.78
20.72 11.82 14.04
18
8
9
51
29
141
8
12
1
0
259 28.78
114.97
15.30 46.08 57.90 28.07
Total
24
24
148
136
783
111
152
28
7
1413
Means
4.00 4.00 24.67 22.67
130.50
18.50
25.33 4.67 1.17
z
Overall G value = 201.34 (40 df); G values of two treatments (column × row) <15.5 nonsignificant at P < 0.05 χ2.
similar to previous findings (Sanxter et al.,
1994). Because differences in external and
internal quality between treatments were not
large, the HSP of <18 h may enable the Hawaiian avocado industry to heat fruit for 8 to 12 h,
with 10 h being optimal, before the quarantine
cold storage at <2.2C pulp for 16 days, with
minimal sacrifice of fruit quality.
HORTS CIENCE, VOL. 30(5), AUGUST 1995
Literature Cited
Animal and Plant Health Inspection Service. 1994.
T107(a): Cold treatment schedules for
Ceratitis capitata (Medfly), p. 5.62. In: Plant
protection and quarantine treatment manual. U.S.
Dept. of Agriculture, Hyattsville, Md.
Liquido, N.J., H.T. Chan, Jr., and G.T. McQuate.
1995. Hawaiian tephritid fruit flies (Diptera):
18
Integrity of the infestation-free quarantine procedure for ‘Sharwil’ avocado. J. Econ. Entomol.
88:85–96.
Sanxter, S.S., K.A. Nishijima, and H.T. Chan, Jr.
1994. Heat-treating ‘Sharwil’ avocado for cold
tolerance in quarantine cold treatments.
HortScience 29:1166–1168.
Sokal, R.R. and F.J. Rohlf. 1969. Biometry. Freeman, San Francisco.
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